At the top of the play, the god Dionysus prances out and tells us he's in disguise as the mortal form of the Stranger. He's come to Thebes to spread his religion. His wild rituals are a big deal all over Asia, but Thebes is the first place in Greece where he's brought them. The ruling family of Thebes, the house of Cadmus, has managed to really tick the god off, by denying his divinity.
Dionysus also fills us in on his whole life story. He's directly related to the main folks who are denying him. His mother, Semele, was part of Cadmus's family. She was also a lover of Zeus. That is, until she was tricked by Zeus's jealous wife, Hera, into asking Zeus to show her his true form. Semele's puny mortal frame couldn't take the king of the gods' super godliness and she was obliterated. Dionysus was still a fetus inside her when this happened. In order to protect his unborn son from Hera, Zeus stitched little Dionysus into his thigh until the child was ready to be born. Since Dionysus's birth, the rest of the royal family has denied that he's a god.
Dionysus is now determined to show them all. He's started by turning his mother's sisters into the Maenads. The basic job description for a Maenad includes: dancing in the woods, drinking wine, breast feeding baby animals, and dismembering whoever gets in their way. Dionysus summons the Chorus, a group of Asian female followers, to dance around the city and sing his glory, while he goes off to the mountains to frolic with the Maenads.
Cadmus, the founder of Thebes, and Tiresias, the famous seer, show up all decked out to worship Dionysus. The two old men are wise and know better than to trifle with a god. King Pentheus, Cadmus's grandson, storms on stage. He's incredibly mad about all the women dancing in the woods, especially since one of them is his own mother, Agave. He declares that he'll stop the spread of this new and terrible religion no matter what. Tiresias and Cadmus tell him he'll be sorry, but the stubborn Pentheus doesn't listen.
Pentheus's men bring in a chained Dionysus. Of course, they have no idea that they've got the actual god on their hands, since Dionysus is in mortal form. They think the Stranger is just the head of this new Dionysian cult. King Pentheus mocks the Stranger and tells him he looks girly. The King locks Dionysus up in jail, but swiftly regrets it. The god summons earthquakes and lightning, flattening his jail and ravaging the palace. A Herdsman toddles on and reports that the Maenads have gotten even crazier. The Herdsman and his buddies tried to take them into custody, and the women went nuts, ripping up cattle and raising hell all over the countryside.
King Pentheus determines that it's time to summon his soldiers and kill all the Maenads. However, Dionysus convinces Pentheus that he ought to go and observe the rituals before he makes his decision. The only way to do this, says Dionysus, is for the King to dress in women's clothing. Pentheus is dubious, but after the god places him in a weird trance, the King is all about cross-dressing. This works out terribly for Pentheus. Dionysus sells him out while he's spying on the Maenads. The ladies go into a frenzy when they see the King. Pentheus ends up being ripped apart by his own mother, Agave.
Agave returns to Thebes, bearing the head of her son. She's so deep in her Dionysian delirium that she thinks she's holding the head of a lion. Her father, Cadmus, knocks some sense into her, and Agave realizes the horrible thing she's done. In the end, Dionysus appears in all his godly glory. He curses Cadmus, saying that he'll be turned into a snake and must lead an army of barbarians in battles against Greece. Agave ends up being banished from Thebes. At the close of the play, the Chorus sings a song of praise for their victorious god, Dionysus.